Communication channels (e.g., wireless, wired and optical), especially wireless channels, exhibit noise which decreases the reliability of the received signal (i.e., the transmit signal is not correctly received at the receiver). Techniques known in the art to increase the reliability of the received signal include Forward Error Corrections (FEC), equalization and transmission diversity. Transmission diversity includes time diversity, delay diversity, frequency diversity and space diversity. In space diversity, copies of the transmit signal propagated via different paths toward the receiver. The receiver combines these copies to increase the received signal power. In frequency diversity, copies of the information signal are modulated over a number of different carrier frequencies. The receiver receives each of the modulated carrier frequencies and combines the received signals. According to the delay diversity technique, a transmitter transmits the same signal several times, each time at a different time-delay. When a transmitter employs a single antenna the antenna transmits the signal over an omni-directional beam of an electromagnetic wave and the transmitter transmits delayed versions of the signal via the single antenna. The transmitter may employ a plurality of antennas (i.e., an array of antennas), and transmit the signal via the antennas, at a time-delay associated with each antenna. When the transmitter transmits the signal via the antennas, at the time-delay associated with each antenna, and the transmissions of the delayed signal overlap, the frequency response of the communication channel (i.e., the attenuation and the phase shift of the channel caused by interference in the channel at different frequencies) may attenuate at certain frequencies where the delayed transmitted signals destructively interfere with each other (i.e., the channel is a frequency selective channel). Furthermore, the time-delay between the transmitted signals (i.e., the signals transmitted by each of the antennas) introduces a phase-shift between the transmitted signals. Thus, instead of an omni-directional beam of an electromagnetic wave, created when a single antenna is used, the antennas create beams which exhibit spatial directionality. This directionality is a result of the destructive and constructive interference of the transmitted signals in space (i.e., similar to a diffraction pattern of a plurality of point light sources). In general, the maximum number of beams produced corresponds to the number of antennas. The number of beams together with the direction, width and length of the beams is referred to herein as the ‘beam pattern’. Due to the spatial directionality of the beams, two receivers for example, located at two different spatial locations relative to the transmitting antennas, may receive the transmitted signal at different received levels of power.
As mentioned above, the direction of the beams is determined according to the relative time-delay or, alternatively, the relative phase-shift between the transmitted signals.
Reference is now made to FIGS. 1A, 1B and 1C. FIGS. 1A and 1B are schematic illustrations of two signals 10 and 12 respectively, with a time difference ΔT there between. FIG. 1C is a schematic illustration of an exemplary transmitter, generally referenced 20, for transmitting a signal using delay diversity. Transmitter 20 includes two antennas 22 and 24 and a beam former 30. Beam former 30 includes two delays 26 and 28. Antenna 22 is coupled with delay 26 and antenna 24 is coupled with delay 28. A signal X is provided to delay 26 and to delay 28. Delay 26 delays signal X by T0 (e.g., T0 is equal to zero in FIG. 1A). Delay 28 delays signal X by T1 wherein T1−T0=ΔT (e.g., T1 is equal to ΔT in FIG. 1B). Antenna 22 transmits the signal delayed by T0 and antenna 24 transmits the signal delayed by T1. As a result of delays introduced to signal X by delays 26 and 28 of beam former 30, the signals transmitted by each antenna undergo constructive and destructive interference. Consequently, system 20 transmits the signal over a beam 32 of electromagnetic waves, where beam 32 exhibits spatial directionality. The direction of beam 32 is determined according to ΔT and the transmitted frequency. Furthermore, beam 32 may exhibit attenuation at certain transmitted frequencies. It is noted that, in fact, system 20 produces two beams, directed in opposite directions however, only one beam is depicted in FIG. 1C.
U.S. Patent application publication 2006/0168165, to van Nee, entitled “Delay Diversity and Spatial Rotation Systems and Methods” is directed towards a system and a method for combining delay diversity and spatial rotation. The system directed to by van Nee includes a Forward Error Correction (FEC) encoder, a puncture module, a spatial stream parser, a plurality of interleavers and a plurality of modulators. The system disclosed by van Nee further includes a cyclic delay module, a Walsh matrix operator, a plurality of Inverse Fast Fourier Transform (IFFT) modules, a plurality of RF/analog modules and a plurality of antennas. Each antenna is coupled with a respective RF/analog module. Each IFFT module is coupled with a respective RF/analog module and with the Walsh matrix operator. The cyclic delay module is coupled with the Walsh matrix operator and with each of the modulators. Each frequency interleaver is coupled with a respective modulator and with the spatial stream parser. The puncture module is coupled with the spatial stream parser and with the FEC encoder.
An input data stream is provided to the FEC encoder which encodes the input data stream to create codewords. The puncture module removes redundant bits from the encoded data stream. The spatial stream parser separates the input data stream into a number of spatial streams. Each frequency interleaver re-orders the bits of the spatial streams such that the transmitted spatial streams are not mirror images of each other. Each modulator modulates the interleaved spatial stream provided by the respective frequency interleaver coupled thereto.
The cyclic delay module introduces to each spatial stream a cyclic delay. The output of the delay modules are cyclically delayed spatial streams. The number of cyclically delayed spatial streams may be different from the number of spatial streams at the input of the cyclic delay module. The Walsh matrix operator introduces a spatial rotation for each cyclically delayed spatial stream thereby mapping each delayed spatial stream to a transmit signal. The IFFT modules combine spatial streams and the sub-carriers into time-domain signals which are used by the RF/analog modules for transmissions by the antennas.